Shrink films, also referred to as heat-shrinkable films, are widely used in both industrial and retail bundling and packaging applications. Such films are capable of shrinking upon application of heat to release stress imparted to the film during or subsequent to extrusion. The shrinkage can occur in one direction or in both longitudinal and transverse directions.
Industrial shrink films are commonly used for bundling articles on pallets. Typical industrial shrink films are formed in a single bubble blown extrusion process to a thickness of about 80 to 200 μm, and provide shrinkage in two directions, typically at a machine direction (MD) to transverse direction (TD) ratio of about 60:40. The main structural component of such industrial shrink films is typically high pressure, low density polyethylene (LDPE), often blended with up to about 30 weight percent of linear low density polyethylene (LLDPE) to reduce problems of hole formation during shrinkage. Typical LDPEs used have a melt index (I2.16) of about 0.2 to 0.5, and typical LLDPEs used have a melt index of about 0.5 to 1.
Retail shrink films are commonly used for packaging and/or bundling articles for consumer use, such as, for example, in supermarket goods. Such films are typically formed in a single bubble blown extrusion process to a thickness of about 35 to 80 μm, with a typical MD:TD shrink ratio of about 80:20. Typical films are formed of LDPE/LLDPE blends, with the LDPE component having a melt index of about 0.5 to 1.
It has been difficult to obtain films combining good optical properties, such as clarity, haze and gloss, good shrink properties, and sufficient holding force, as well as good mechanical properties such as puncture resistance and tear strength. Monolayer films made predominantly of LDPE have poor optical properties, particularly when using a low melt index resin. Moreover, during shrinkage, the shrink force can be excessive, leading to ready formation of holes. In practice, such films need to be thicker than would be desired, to off-set such undesirable characteristics. Alternatively, small amounts of LLDPE can be added to improve optical and mechanical properties, but if more than about 30 wt. % LLDPE is used, the shrinkage becomes insufficient, and the processing requirements for film blowing become too demanding.
Another conventional approach to improving film properties of LDPE/LLDPE blends has been the development of double bubble equipment and processes to provide additional transverse direction stretch to the film. Such equipment forms the film in two successive bubbles, with an intermediate heating step between the two bubbles. In this way, higher amounts of LLDPE can be used while still maintaining adequate shrink properties. Unfortunately, such processes are complex and costly, and the specialized equipment requires a significant capital investment.
Thus, it would be desirable to have heat shrinkable films with a combination of good optical, mechanical and shrink properties, without resort to expensive and complex processing equipment.